Line drivers that fits within a specified line pitch

ABSTRACT

Line drivers that fit within a specified line pitch. One method of placing line drivers completely underneath a cross point array requires splitting the line driver up so that a portion of the line drivers is on a first side of the cross point array and the other portion is on the opposite side. However, using this technique requires that the width of the drivers is no larger than the width of the memory cells that are being driven. This can be accomplished by stacking transistors such that line drivers fit within a specified line pitch, but are as long as is necessary to include all the necessary circuitry.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.10/613,099, filed Jul. 1, 2003, now U.S. Pat. No. 6,836,421, whichclaims the benefit of U.S. Provisional Application No. 60/400,849, filedAug. 2, 2002, the U.S. Provisional Application No. 60/422,922, filedOct. 31, 2002, and the U.S. Provisional Application 60/424,083, filedNov. 5, 2002, all of which are incorporated herein by reference in theirentireties and for all purposes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to memory, and more specificallyto memory employing a cross point array.

2. Description of the Related Art

A random access memory (“RAM”) type of memory is typically associatedwith the main memory available to computer programs and similar devices.RAM type memory is often contrasted with a read-only memory (“ROM”) typeof memory, which is typically associated with a special memory that iseither not changed, or changed relatively infrequently. RAM mostlycomprises SRAM and DRAM. ROM mostly comprises Flash memory, EPROM, OTP,EEPROM, PROM and ROM. Some devices such as NovRAM and Battery poweredSRAM are hybrid devices using more than one technology.

Although SRAM is the memory of choice for computer applications, withvery fast access times, its volatility, large size and stand-by currentlimit the total size and applications of the memory. Nonvolatilememories such as Flash memory are slower to program, and in some casemust be erased a large block at a time before being reprogrammed. DRAMhas the smallest cell size, but necessitates a complex refreshalgorithm, and is volatile. For new applications, away from PCapplications and into portable applications such as cell phones,personal digital assistants (PDA), digital cameras, camcorders,removable “key-chain” or “USB” disks, the key issues are nonvolatilityand low power consumption.

Regardless of how the memory is used, RAM and ROM overlap in manyrespects. Both types of memory can allow random access reads. Both typesof memory can be relatively fast or relatively slow. Although all ROMsare non-volatile, so are some RAMs. Although most ROMs cannot changetheir data once programmed, some ROMs can be re-programmed. RAM,however, is always re-writable.

The ROMs that are capable of modifying their data typically require longwrite cycles that erase entire blocks of data prior to new data beingwritten. For example, UV light might be applied to an entire memoryblock in order to “repair” fused connections so that the block can bere-written with new data. RAM, on the other hand, can read or write to arandomly accessed byte of memory, typically performing either operationin a standard cycle.

Conventional nonvolatile RAM and ROM require three terminal MOSFET-baseddevices. The layout of such devices are not ideal, usually requiringfeature sizes of at least 8 f² for each memory cell, where f is theminimum feature size.

However, not all memory elements require three terminals. Certaincomplex metal oxides (CMOs), for example, can retain a resistive stateafter being exposed to an electronic pulse, which can be generated fromtwo terminals. U.S. Pat. No. 6,204,139, issued Mar. 20, 2001, to Liu etal., incorporated herein by reference for all purposes, describes someperovskite materials that exhibit such characteristics. The perovskitematerials are also described by the same researchers in“Electric-pulse-induced reversible resistance change effect inmagnetoresistive films,” Applied Physics Letters, Vol. 76, No. 19, 8 May2000, and “A New Concept for Non-Volatile Memory: The Electric-PulseInduced Resistive Change Effect in Colossal Magnetoresistive ThinFilms,” in materials for the 2001 Non-Volatile Memory TechnologySymposium, all of which are hereby incorporated by reference for allpurposes.

Similarly, the IBM Zurich Research Center has also published threetechnical papers that also discuss the use of metal oxide material formemory applications: “Reproducible switching effect in thin oxide filmsfor memory applications,” Applied Physics Letters, Vol. 77, No. 1, 3Jul. 2000, “Current-driven insulator-conductor transition andnonvolatile memory in chromium-doped SrTiO₃ single crystals,” AppliedPhysics Letters, Vol. 78, No. 23, 4 Jun. 2001, and “Electric currentdistribution across a metal-insulator-metal structure during bistableswitching,” Journal of Applied Physics, Vol. 90, No. 6, 15 Sep. 2001,all of which are hereby incorporated by reference for all purposes.

Similarly, magnetic RAM (MRAM) requires only two terminals to deliver amagnetic field to the memory element. Other two terminal devices includeOvonic Unified Memory (OUM), which uses chalcogenic layers of material,and various types of ferroelectric memory. With only two terminals, ithas been theorized that memory can be arranged in a cross pointarchitecture.

However, mere recognition that a two terminal memory element istheoretically capable of being placed in a cross point array does notsolve many of the non-trivial problems associated with actually creatingsuch a device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may best be understood by reference to the followingdescription taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 depicts a perspective view of an exemplary cross point memoryarray employing a single layer of memory;

FIG. 2 depicts a side view of an exemplary memory plug with five layersand sandwiched between two conductive array lines;

FIG. 3A illustrates selection of a memory cell in the cross point arraydepicted in FIG. 1;

FIG. 3B illustrates the boundaries of the selected memory cell of FIG.3A;

FIG. 4A depicts a perspective view of an exemplary stacked cross pointmemory array employing four layer of memory;

FIG. 4B depicts a schematic diagram of the stacked cross point memory ofFIG. 4A;

FIG. 5 depicts a cross section of the stacked cross point array of FIG.4A;

FIG. 6A depicts a schematic diagram of x-direction driver sets inconjunction with the stacked cross point memory of FIG. 4A;

FIG. 6B depicts a schematic diagram of y-direction driver sets inconjunction with the stacked cross point memory of FIG. 4A;

FIG. 7A depicts the general layout of the x-direction and y-directiondriver sets depicted in FIGS. 6A and 6B in relation to the stacked crosspoint array depicted in FIG. 4A;

FIG. 7B depicts an abstracted representation of FIG. 7A;

FIG. 8A depicts the general layout of interdigitated x-direction andy-direction driver in relation to the single layer cross point arraydepicted in FIG. 1;

FIG. 8B depicts an abstracted representation of FIG. 8A;

FIG. 9A depicts the general layout of interdigitated x-direction andy-direction driver in relation to the stacked cross point array depictedin FIG. 4A;

FIG. 9B depicts an abstracted representation of FIG. 8A;

FIGS. 10A and 10B depicts a schematic diagram of alternating x-directiondriver sets in conjunction with the stacked cross point memory of FIG.4A;

FIGS. 11A through 11C depict various layouts of the x-direction andy-direction driver sets that have some drivers underneath thesingle-layer cross point array depicted in FIG. 1;

FIG. 12 depicts a layout of the x-direction and y-direction driver setsthat interdigitate some drivers but not others such that some driversare underneath the single-layer cross point array depicted in FIG. 1;

FIG. 13A depicts a layout of an x-direction driver set and a y-directiondriver set that completely fits underneath the single-layer cross pointarray depicted in FIG. 1;

FIG. 13B depicts another layout of an x-direction driver set and ay-direction driver set that completely fits underneath the single-layercross point array depicted in FIG. 1;

FIG. 14A depicts a layout of x-direction driver sets and a y-directiondriver sets that completely fits underneath the stacked cross pointarray depicted in FIG. 4A;

FIG. 14B depicts a layout of x-direction driver sets and a y-directiondriver sets that leaves empty gaps underneath the single-layer crosspoint array depicted in FIG. 4A;

FIG. 15A depicts a schematic diagram of x-direction driver sets inconjunction with the stacked cross point memory of FIG. 4A;

FIG. 15B depicts a schematic diagram of y-direction driver sets inconjunction with the stacked cross point memory of FIG. 4A;

FIG. 16 depicts an alternative schematic diagram of x-direction driversets in conjunction with the stacked cross point memory of FIG. 4A thatuses separate drivers that operate on identical logic;

FIG. 17 depicts an alternative schematic diagram of y-direction driversets in conjunction with the stacked cross point memory of FIG. 4A thateliminates the need for additional metal layers;

FIG. 18A depicts a layout of an x-direction driver set and a y-directiondriver set that connects to the single-layer cross point array depictedin FIG. 1;

FIG. 18B depicts a layout of multiple x-direction driver sets andy-direction driver sets that connects to the stacked cross point arraydepicted in FIG. 4A;

FIG. 19 depicts a layout of the X0 driver set that fit within thenecessary line pitch;

FIG. 20A depicts a driver schematic in which the driver requires accessto a primary decoder, a secondary decoder, and a reference voltage;

FIG. 20B depicts a layout of the three transistors depicted in FIG. 20A;

FIG. 21A depicts the driver depicted in FIG. 20B after a first metallayer is deposited;

FIG. 21B depicts the driver depicted in FIG. 21A after a second metallayer is deposited;

FIG. 22 depicts a layout of the last stage of drivers;

FIG. 23A depicts the driver depicted in FIG. 22 after a first metallayer is deposited;

FIG. 23B depicts the driver depicted in FIG. 23A after a second metallayer is deposited; and

FIG. 24 depicts a schematic diagram of a stacked cross point memoryarray employing eight layer of memory.

It is to be understood that, in the drawings, like reference numeralsdesignate like structural elements. Also, it is understood that thedepictions in the figures are not necessarily to scale.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, numerous specific details are set forth toprovide a thorough understanding of the present invention. It will beapparent, however, to one skilled in the art that the present inventionmay be practiced without some or all of these specific details. In otherinstances, well known process steps have not been described in detail inorder to avoid unnecessarily obscuring the present invention.

Overview

The random access reads and writes that are required for RAM devicestypically require that a particular memory cell be capable of beingselected out of an entire memory block. For a two terminal memory cellin a cross point structure, that usually means that both terminals areused for selection. Since one of the terminals is common to an entirerow of memory cells and another terminal is common to an entire columnof memory cells in a cross point array, writing with a single terminalwould result in writing to either an entire row or an entire column.Therefore, a single cross point array would typically have both anx-direction driver set and a y-direction driver set.

Compared to a memory cell in a cross point array, driver sets are fairlylarge, typically requiring several transistors to drive a single columnor row. Although the cross point array can be fabricated over all thedriver circuitry, placing the drivers underneath the cross point arraywhile conserving space is not a trivial matter. One technique that canbe used to ensure an efficient use of space is by splitting the linedriver up so that a portion is on a first side of the cross point arrayand the other portion is on the opposite side. However, using thistechnique requires that the width of the driver is no larger than thewidth of the memory cells that are being driven.

The Cross Point Array

FIG. 1 depicts an exemplary cross point array 100 employing a singlelayer of memory. A bottom layer of x-direction conductive array lines105 is orthogonal to a top layer of y-direction conductive array lines110. A plurality of memory plugs 115 are located at the intersections ofthe conductive array lines, each individual memory plug being capable ofbeing uniquely identified and, therefore, uniquely selected by a singlex-direction conductive array line and a single y-direction conductivearray line.

Conductive array line layers 105 and 110 can be constructed of anyconductive material, such as aluminum, copper, tungsten or certainceramics. Depending upon the material, a conductive array line wouldtypically cross between 64 and 8192 perpendicular conductive arraylines. Fabrication techniques, feature size and resistivity of materialmay allow for shorter or longer lines. Although the x-direction andy-direction conductive array lines can be of equal lengths (forming asquare cross point array) they can also be of unequal lengths (forming arectangular cross point array).

The memory plug layer 115 will typically dictate the requirements of atleast the conductive array line layer 105 beneath the memory plug layer115. Certain fabrication process (e.g., solution based spin on followedby high temperature anneal, pulsed laser deposition, sputtering, andmetalorganic chemical vapor deposition) might require that refractorymetals be used for conductive array lines so that they may withstand thehigh temperature fabrication process. However, refractive array lineshave higher resistances, which means that a given x-direction conductivearray line would not be able to timely access as many memory plugs as alower resistance conductive array line, reducing the number of memorycells on the cross-point array 100.

Each memory plug in the memory plug layer 115 contains a memory elementalong with any other materials that may be necessary. The additionalmaterials might include a non-ohmic device, as is described inco-pending application “High Density NVRAM,” U.S. application Ser. No.10/360,005, filed Feb. 7, 2003, already incorporated by reference. Thenon-ohmic device exhibits a very high resistance regime for a certainrange of voltages (V_(NO−) to V_(NO+)) and a very low resistance regimefor voltages above and below that range. Together, the memory elementand the non-ohmic device cause the memory plug to exhibit a non-linearresistive characteristic.

FIG. 2 depicts a side view of an exemplary memory plug 205 with fivelayers and sandwiched between two conductive array lines 210 and 215.The five layers are: a first electrode layer 220, a layer ofmulti-resistive state material 225, a second electrode layer 230, alayer making up the non-ohmic device 235, and a third electrode 240.Some layers may actually be made up of multiple thin films. For example,one type of non-ohmic device 235 uses a three film metal-insulator-metal(MIM) structure. Additionally, not all the layers are required for everymemory plug 205 configuration. For example, certain cross point arraysmay use means other than the non-ohmic device 235 to prevent unselectedmemory plugs from being disturbed.

Typical electrodes 220, 230 and 240 commonly used in fabrication includePt, Au, Ag and Al. If the only purpose of the electrodes 220, 230 and240 is as a barrier to prevent metal inter-diffusion, then a thin layerof metal, e.g. TiN, could be used. If a seed layer is additionallyrequired, any number of electrically conductive materials can be usedfor on top of the thin layer of metal. For example, the seed layer couldbe a conductive perovskite, such as LaNiO₃ or SrRuO₃ on Pt, a conductivemetal oxide, such as IrO₂ on Ir or RuO₂ on Ru, a noble metal such as Pton TiN. It will be appreciated that the choice of electrode layers 220,230 and 240 in combination with the multi-resistive state material layer225 may affect the properties of the memory element. As such, the memoryfunction is realized either by the multi-resistive state material 225properties or by the interface between an electrode 220 or 230 and themulti-resistive state material 225.

The multi-resistive state material 225 will generally be a crystallineor polycrystalline structure. One class of multi-resistive statematerial 225 are perovskites that include two or more metals, the metalsbeing selected from the group consisting of transition metals, alkalineearth metals and rare earth metals. The perovskites can be any number ofcompositions, including manganites (e.g., Pr_(0.7)Ca_(0.3)MnO₃,Pr_(0.5)Ca_(0.5)MnO₃ and other PCMOs, LCMOs, etc.), titanates (e.g.,STO:Cr), zirconates (e.g., SZO:Cr, Ca₂Nb₂O₇:Cr, and Ta₂O₅:Cr), and highTc superconductors (e.g., YBCO). Specifically, MnO₃, when combined withthe rare earth metals La, Pr or some combination thereof and thealkaline earth metals Ca, Sr or some combination thereof have been foundto produce a particularly effective multi-resistive state material 225for use in the memory plug 205. The compounds that make up this class ofmulti-resistive state materials 225 include both normal conductive metaloxides and conductive complex metal oxides.

Multi-resistive state materials 225, however, are not limited toperovskites. Specifically, any material that uses trapped charges tomodify or alter conductivity could be used in the memory plug 205.Doping different materials (e.g., insulators, conductors, conductiveoxides, and/or polymers), for example, may create charge traps bysubstituting the dopant for crystalline elements. Doping may also beused to create charge traps by interstitially introducing dopants into acrystalline structure. Also, introducing separate physical clusters, orgroups of atoms, into a crystalline structure may create charge traps aswell.

While the resistance changing effect of the multi-resistive statematerial 225 is generally not dependent upon its area or its thickness,the resistance value of the multi-resistive state material 225 isdependant on both its area and its thickness as well as otherproperties. Similarly, the voltage at which multi-resistive statematerial 225 switches resistive states is also dependent upon thevarious properties of the material.

The various properties of the multi-resistive state material 225 willdetermine both the lowest possible resistance state and the highestpossible resistive state. Although the highest operational resistivestate (R₀) of the memory plug 205 does not need to be its highestpossible state and the lowest operational resistance state (R₁) does notneed to be its lowest possible state, designs can set R₀ and R₁ close tothose states for simplicity.

When considering an operative value of the R₁ resistive state parasiticresistances that are in series with the memory plug must also beconsidered. Sources of resistance include the contacts and the vias, themetal interconnect lines, and the driver circuits. Parasitic resistancesmight total 100–200Ω, which puts a lower limit on the memory plugresistance. While the parasitic resistance may be overcome through morecomplicated circuit design, such designs typically result in loss ofaccess time, or a larger die size.

The R₁ state of the memory plug may have a best value of 10 kΩ to 100kΩ. If the R₁ state resistance is much less than 10 kΩ, the currentconsumption will be increased because the cell current is high, and theparasitic resistances will have a larger effect. If the R₁ state valueis much above 100 kΩ, the RC delays will increase access time. However,workable single state resistive values may also be achieved withresistances as low as 5 kΩ and as high as 1 MΩ. Typically, a singlestate memory would have the operational voltages of R₀ and R₁ separatedby a factor of 10.

For example, if 1 volt were used as a read voltage (V_(R)), R₁ might beabout 100 kΩ and R₀ might be about 1MΩ, making the current either 10 μAor 1 μA, depending on the resistive state. Since large currents can bedestructive to semiconductors fabricated to small dimensions, no morethan 10 μA would be desired for a memory circuit in most cases. Once aV_(R) is identified, a desired write voltage (V_(W)) can also bedetermined. Not only should V_(W) be greater than V_(R), but it shouldalso be far enough away from V_(R) to allow minor voltage fluctuations(e.g., due to fabrication imperfections) to have a negligible effect onthe multi-resistive state material 225. Similarly, V_(W) should begreater than V_(Wth), the threshold at which the resistive materialstarts to change resistivity, for the same reason. A typical V_(W) mightbe about 2 volts, and V_(Wth) might be about 1.5 volts.

The properties of the non-ohmic device 235 would then be dependant on ½V_(R). FIG. 3A illustrates selection of a memory cell 315 in the crosspoint array 100. The point of intersection between a single x-directionconductive array line 305 and a single y-direction conductive array line310 uniquely identifies a single memory cell 315. FIG. 3B illustratesthe boundaries of the selected memory cell 315. The memory cell arepeatable unit that can be theoretically extended in all dimensions. Incertain embodiments, the memory cells are repeated in the z direction(orthogonal to the x-y plane). A preferred method of repeating thememory cells in the z-direction is to use both the bottom and topsurfaces of conductive array lines that sandwich memory plugs layers.Therefore, the repeatable cell that makes up the array of memory cellscan be considered to be a memory plug, plus ½ of the space around thememory plug, plus ½ of an x-direction conductive array line and ½ of ay-direction conductive array line. Of course, ½ of a conductive arrayline is merely a theoretical construct, since a conductive array linewould generally be the same width, regardless of whether both surfacesof the conductive array line was used. Accordingly, the very top andvery bottom layers of conductive array lines (which use only onesurface) would typically be fabricated to the same size as all otherlayers of conductive array lines.

During selection the selected conductive array lines 305 and 310 delivera voltage to every memory cell associated with those conductive arraylines. Therefore, the non-ohmic device must be able to pass current fromthe minimum voltage seen on a selected memory cell 315 but block atleast some current to unselected memory cells. If each conductive arrayline 305 and 310 supplies half the required voltage of each operation,then ½ V_(W)′ would be the maximum voltage on a single conductive arrayline and V_(R)′ would be the minimum voltage seen on a selected memorycell 315. V_(W)′ is V_(W) plus all other voltage drops in the memorycell 315 (e.g., the voltage drop across the non-ohmic device V_(NO+))and V_(R)′ is V_(R) plus all other voltage drops in the memory cell 315.Accordingly, the selection of only one conductive layer is notsufficient to access a single memory cell array for writing purposes. Inembodiments where ½ V_(R)′ is applied by both conductive array lines,the selection of a single conductive layer is also not sufficient toaccess a single memory layer for reading purposes. However, somedecoding circuitry might supply the full read voltage from oneconductive array line, then read data from the entire memory layer andsimply discard data from unwanted lines.

It should be noted that changes in the resistive property of the memoryplugs that are greater than a factor of 10 might be desirable inmulti-bit resistive memory cells. Generally, adjusting the pulse widthand magnitude of the voltage pulses across the multi-resistive statematerial results in multiple resistive states. Since multi-resistivestate material 225 can be placed into several different resistivestates, multi-bit resistive memory cells are possible. For example, themulti-resistive state material might have a high resistive state of R₀₀,a medium-high resistive state of R₀₁, a medium-low resistive state ofR₁₀ and a low resistive state of R₁₁. Since multi-bit memories typicallyhave access times longer than single-bit memories, using a factorgreater than a 10 times change in resistance from R₁₁ to R₀₀ is one wayto make a multi-bit memory as fast as a single-bit memory. For example,a memory cell that is capable of storing two bits might have the lowresistive state be separated from the high resistive state by a factorof 100. A memory cell that is capable of storing three or four bits ofinformation might require the low resistive state be separated from thehigh resistive state by a factor of 1000. Typically, the intermediaryresistive states in a multi-bit memory would evenly subdivide theresistive range between the high resistive state and the low resistivestate on a logarithmic scale. For example, if a memory cell that heldthree bits of memory had a low resistive state of 10 kΩ, the sixintermediary states might have resistive states of about 26.8 kΩ, 72.0kΩ, 193 kΩ, 518 kΩ, 1.39 MΩ, and 3.73 MΩ. The highest resistive statewould then be 10 MΩ, 1000 times the value of the low resistive state.Each optimal resistive state could easily be calculated by using therelationship Log(R₁₁₀)=Log(R₁₁₁ )+Log K; Log(R₁₀₁)=Log(R₁₁₁)+2 Log K;Log(R₁₀₀)=Log(R₁₁₁)+3 Log K; . . . Log(R₀₀₀)=Log(R₁₁₁)+7 Log K, whereLog K=( 1/7)[Log(R₀₀₀)−Log(R₁₁₁)].

Those skilled in the art will appreciate that the above-describedvoltages are voltages seen by the memory plug 315, and not necessarilythe absolute values of the voltages from ground. For example, placing 10volts on the selected x-direction conductive array line 305 and 6 voltson the selected y-direction conductive array line 310 would still attaina 4-volt drop across the memory plug 315. If the voltage drop across anysingle unselected memory plug was to be no more than 3 volts, thenunselected conductive array lines 320, 325, 330, 335 and 340 would needto be held somewhere between 7 volts and 9 volts in the above example.An opposite polarity voltage drop would then be attained in the aboveexample whenever the selected x-direction conductive array line 305 washeld to a smaller voltage than the selected y-direction conductive arrayline 310.

In many designs the unselected conductive array lines 320, 325, 330, 335and 340 would be held at a constant voltage, regardless of whether aread or write operation was being performed. Therefore, if the selectedx-direction conductive array line 310 were placed at 12 volts, then they-direction unselected conductive array lines 320 and 325 would need tobe held at 9 volts in the above example in order to keep the maximumvoltage drop to 3 volts. If symmetry was desired, a write operationmight require the x-direction conductive array line 310 be held to 11volts, the y-direction conductive array line 305 be held to 5 volts andall the unselected conductive array lines 320, 325, 330, 335 and 340 beheld to 8 volts (halfway between the voltages of the two conductivearray lines 305 and 310 for both read and write operations).

Ideally, the multi-resistive state material 225 should switch veryquickly from one resistive state to another. Typically, anything lessthan 50 nanoseconds would be an appropriate switching speed for currentapplications such as mobile phones, PDAs or other portable electronicsdevices. Additionally, once the multi-resistive state material 225 isplaced in a resistive state, it should be able to retain that state forlong periods of time. Ideally, the material should retain its resistivestate for over ten years. Since the read voltage should not affect theresistive state, repeated application of the read voltage over ten yearsshould not change the resistive state of the multi-resistive statematerial 225.

Generally, the chemical and materials properties of the multi-resistivestate material 225 are selected to meet the electrical specificationsset forth above. For example, the material preferably has a resistivityof not greater than about 1 Ω-cm (although 5 Ω-cm may be a workablevalue), exhibits a change in resistance of at least about 10×, and hasthis resistance change triggered by the application of a voltage pulseof not longer than about 100 ns duration and not greater than about 10Vin magnitude. In addition, the memory element material should becompatible with the requirements imposed by the general fabricationprocess of the entire high density RAM.

The Stacked Cross Point Array

FIGS. 4A and 4B depict an exemplary stacked cross point array 400employing four memory layers 405, 410, 415, and 420. The memory layers,ML₀ 405, ML₁ 410, ML₂ 415 and ML₃ 420, are sandwiched betweenalternating layers of x-direction conductive array lines, X₀ layer 425,X₁ layer 430, and X₂ 435, and y-direction conductive array lines, Y₀layer 440 and Y₁ layer 445. Stacking multiple memory layers is onetechnique that can be used increase the memory capacity of a memorydevice employing a cross point array. Additionally, stacking allows forthe memory device to increase its memory capacity without increasing thefootprint of the cross point array 400.

Each memory layer 405, 410, 415, and 420 is associated with one layer425, 430, or 435 of x-direction conductive array lines and one layer 440or 445 of y-direction conductive array lines. Although the topconductive array line layers 425 and bottom conductive array line layer435 are only used to supply voltage to a single memory layer 405 and420, the other conductive array line layers 430, 440, and 445 can beused to supply voltage to both a top and a bottom memory layer 405, 410,415, or 420.

Generally speaking, if each middle conductive array line layer were usedfor two memory layers, N memory layers would require N+1 layers ofconductive array lines. However, not every conductive line layer wouldneed to have its own separate circuitry. Since selection of a memoryplug requires both an x-direction conductive array line and ay-direction conductive array line to be active, multiple conductivearray lines in both directions can be activated as long as only onememory layer can be activated.

For example, in the stacked cross point array 400, the top and bottomx-direction layers of conductive array lines (the X₀ layer 425 and theX₂ layer 435) can be logically related for accessing memory elements,and, in addition to sharing the same selection logic, may even share thesame circuitry. Table 1 shows the combinations of x-direction conductivearray lines and y-direction conductive array lines that are necessary toactivate a memory element on a specific memory layer.

TABLE 1 X₀, X₂ X₁ Y₀ Y₁ Memory Layer x x ML₀ x x ML₁ x x ML₂ x x ML₃

Therefore, in order to access a memory cell on ML₁ 410, for example, oneconductive array line from the X₁ layer 430 and one conductive arrayline from the Y₀ layer 440 would need to be selected.

Activating the appropriate conductive array lines is accomplishedthrough peripheral circuitry. Typically, the peripheral circuitry wouldbe drawn on a semiconductor substrate underneath the stacked cross pointmemory array 400. Thrus, which are similar to stacked vias, are onemechanism that can be used to reach the peripheral circuitry.

FIG. 5 depicts the exemplary stacked cross point array 400 employingthrus 505 and 510 for the two upper layers of x-direction conductivearray lines 430 and 435. As will be appreciated by those skilled in theart, similar thrus could also be used to connect the peripheralcircuitry to the y-direction conductive array lines 440 and 445.

The thru 505 that connects the X₁ layer 430 to the peripheral circuitrywould go through at least two inter-layer dielectric (ILD) layers 515and 520. ILD layers provide several functions, including preventingcapacitive coupling between conductive array lines and providing asubstrate for conductive array lines that would otherwise need to spanempty spaces.

The thru 510 that connects the X₂ layer 435 to the peripheral circuitrywould go through at least four ILD layers 515, 520, 525, and 530.Although thrus 505 and 510 may be formed during the same processingsteps as the memory layers, a conductive material 535 would need to beused to connect the conductive array lines to the peripheral circuitry.

The Peripheral Circuitry

FIG. 6A depicts x-direction driver sets 605, 610, and 615 that are usedto select specific x-direction conductive array lines in the X₀ layer425, X₁ layer 430, and X₂ layer 435. Although the X₀ driver 605 and theX₂ driver 615 can use identical logic (as described in table 1),separate drivers are shown because of the difficulty in routing thesingle X₀ driver 605 around the thru 505 that connects the X₁ layer 430to the X₁ driver 610.

FIG. 6B depicts y-direction driver sets 620 and 625 that are used toselect specific y-direction conductive array lines in the y-directionconductive array line layers 440 and 445. The Y₀ driver set 620 uses athru 630 that goes through one ILD layer 515 in order to connect withthe Y₀ layer 440. The Y₁ driver set 625 uses a thru 635 that goesthrough three ILD layers 515, 520, and 525 in order to connect with theY₁ layer 445.

FIG. 7A depicts the general layout of the x-direction and y-directiondriver sets 605, 610, 615, 620, and 625 in relation to the stacked crosspoint array 400. FIG. 7B is a more abstracted representation of FIG. 7A.Since each driver set is on the same side the entire layout forms anon-symmetrical L-shape.

However, certain designs may interdigitate the driver circuitry so thatthe driver that controls one conductive array line comes from one sideand the driver that controls the next conductive array line comes fromthe opposite side.

Interdigitated Driver Sets

FIGS. 8A and 8B depict a layout of an interdigitated x-direction driver805 and an interdigitated y-direction driver 810 that drive asingle-layer cross point array 100. Interdigitating the drivers 805 and810 not only allows for greater symmetry in a single-layer cross pointarray 100, but also permits the drivers to be fabricated to thickerdimensions. Interdigitation can additionally be used for stacked crosspoint arrays.

Drivers can be considered to be interdigitated when alternating lines inthe same conductive array line layer are driven from differentlocations. For example, even numbered lines can be driven from one sideand odd numbered lines can driven from the opposite side. Conductivearray lines can also be interdigitated in groups, for example havingSuch oppositely driven lines may be alternate, i.e. alternately a linedriven from one side and a line driven from the other side, or groupedby pair, where 2 lines driven from one side are adjacent and two linesdriven from the other side are next to them, or grouped in anyarrangement which allows the use of a driver which is laid out in thepitch of 2×N cells, and drives N lines. By extension, we call“interdigitated driver set” a set of drivers driving interdigitatedlines even though the drivers themselves are not interdigitated.

FIGS. 9A and 9B depict a layout of driver sets 605, 610, 620, and 625for the stacked cross point array 400 that are interdigitated. Theconfiguration of the x-direction driver sets 605 and 610 are depicted inFIGS. 10A and 10B. The configuration of FIG. 10A is alternated with theconfiguration of FIG. 10B for each x-direction conductive array line sothat both the X₀ driver 605 and the X₁ driver 610 alternate sides.

Additionally, by placing the thru 505 that connects the X₁ layer 430 tothe X₁ driver 610 and thru 510 that connects the X₂ layer 435 to the X₀driver 605 on opposite sides, the bottom X₀ layer 425 can be directlytied to the X₂ layer 435. Therefore, a single X₀ driver 605 can be usedfor both the bottom X₀ layer 425 and the top X₂ layer 435.

The layouts shown in FIGS. 7B, 8B and 9B assume that the drivers 605,610, 615, 620, 625, 805, and 810 all lie outside the cross point array100 or 400. However, if the substrate underneath the cross-point array100 or 400 is used to draw the peripheral circuitry, the footprint ofthe entire memory chip can be reduced.

FIGS. 11A through 11C depict various layouts of the x and y directiondriver sets 805 and 810 that have some drivers underneath thesingle-layer cross point array 100. In each layout, some drivers areleft outside of the single-layer cross point array 100 even though theremay be unused space underneath the cross point array 100. In FIG. 11Athe layout forms a rectangle that extends beyond the cross point array100 in the x-direction. In FIG. 11B the layout forms an H-shape thatextends beyond the cross point array 100 in the y-direction. In FIG. 11Cthe layout forms a spiral shape that, while extending beyond the crosspoint array 100 in both the x-direction and y-direction, forms a moresymmetrical pattern.

FIG. 12 depicts an alternative layout of the x and y direction driversets 805 and 810 that interdigitate some drivers but not others.Portions of each driver set 805 and 810 are still outside thesingle-layer cross point array 100. However, assuming each driver isfabricated to common dimensions, a portion of the non-interdigitateddriver would still extend beyond the cross point array 100, because noninterdigitated drivers drive twice as many lines in the same pitch, andtherefore necessitate more circuitry.

However, if none of the drivers 805 and 810 are interdigitated, then theentire peripheral circuitry might be placed underneath the single-layercross point array 100, as shown in the following paragraphs.

Non-Interdigitated Driver Sets

FIG. 13A depicts a layout of an x-direction driver 805 and a y-directiondriver 810 that completely fits underneath the single-layer cross pointarray 100. FIG. 13B depicts an alternate layout of the x-directiondriver 805 and the y-direction driver 810 that uses a more symmetricallayout.

The concept of using non-interdigitated driver sets can be extended tostacked cross point arrays. FIGS. 14A and 14B depict layouts ofnon-interdigitated driver sets 605, 610, 620, and 625. It should beappreciated that each driver set 605, 610, 620, or 625, is notnecessarily to scale, and, depending upon both the number of lines ineach conductive array line layer and the circuitry used for each driver,each driver set 605, 610, 620, or 625, may completely fit underneath thestacked cross point array 400, or may leave empty gaps underneath thestacked cross point array 400, as shown in FIG. 14B.

FIGS. 15A and 15B depicts one possible configuration of the driver sets605, 610, 620 and 625 that can be used in the layout of FIG. 14A or 14B.While the X₀ driver set 605 can directly connect with the X₀ layer 425(and the X₂ layer 435 via the thru 510) and the X₁ and Y₁ driver sets610 and 625 can be directly connected to their X₁ and Y₁ layers 430 and445 via their respective thrus 505 and 635, the Y₀ driver set 620 willnot be able to directly connect to the thru 630 that connects theperipheral circuitry to the Y₀ layer 440. Instead, the Y₀ driver set 620uses an extra metal layer 1505 in the peripheral circuitry portion inorder to connect to the thru 630 by going over the circuitry of the Y₁driver set 625. The extra metal layer 1505 is in addition to any metallayers that may be necessary to construct the driver circuitry.

FIG. 16 depicts an alternate configuration for the x-direction driversets 605, 610, and 615. Since the extra metal layer 1505 is used inconnection with the y-direction drivers 620 and 625, the configurationof FIG. 15 also utilizes the extra metal layer 1505. By separating theX₀ driver set 605 from the X₂ driver set 615, the load on the X₀ driveris reduced even if the two drivers 605 and 615 use identical logic. Thiswill improve the access time to layer 425 and 435.

FIG. 17 depicts yet another configuration for the y-direction driversets 620 and 625 that can be used to eliminate the need for the extrametal layer 1505. In this configuration, one x-direction conductivearray line is eliminated from the X₀ layer 425, creating an array cutthat allows the Y₀ driver set 620 to directly connect with the thru 630that connects the Y₀ driver set 620 to the Y₀ layer 440. While one lineof memory plugs would be eliminated from the ML₀ layer 405, theirabsence would represent a very small percentage of the total memoryplugs in the stacked cross point array 400 and may be a fair tradeofffor a peripheral circuitry design that does not use an extra metal layer1505. Cells in layers ML₁ 410, ML₂ 415, and ML₃ 420 straight above theeliminated ML₀ array line can either be eliminated, or be kept as nonfunctional dummy cells. This way, the four array layers are symmetrical.

Additionally, if the Y₀ driver set 620 is moved to the middle of thestacked cross point array 400, the access time for the Y₀ layer 440 isimproved. If the Y₀ driver set 620 is at one end of the conductive arraylines then current would need to travel to the opposite end in order toreach the active memory cell in the worst case scenario. However, if theY₀ driver set 620 is positioned in the middle of the conductive arraylines, the current would only need to travel half the length of theconductive array line in the worst case scenario. Access time is,therefore, improved by having the Y₀ driver set 620 positioned in themiddle of the conductive array lines.

FIG. 18A depicts a layout of an x-direction driver 805 and a y-directiondriver 810 that are both centered underneath the single-layer crosspoint array 100. The y-direction driver 810 can use the array cut toreach the middle of the y-direction conductive array lines 110 and thex-direction driver can be positioned anywhere under the x-directionconductive array lines 105 as long as it does not overlap with anotherdriver. Moving both drivers 805 and 810 substantially to the middle oftheir respective conductive array lines 105 and 110 further reduces theaccess time to the memory plug layer 115.

Such a layout would be most effective for either a single layer crosspoint array or the bottom layer of a stacked cross point array. In thecase of a stacked cross point array, while the X₀ driver set 605 and Y₀driver set 620 are positioned substantially in the middle of cross pointarray 400, the other driver sets would typically connect to the ends ofthe conductive array lines for the upper conductive array line layers.

FIG. 18B depicts a layout of driver sets for a stacked cross point array400 where the bottom memory layer has a faster access time than uppermemory layers. Certain applications might require that a portion ofmemory be accessed faster than other portions. For example a massstorage device might want to be able to access its file allocationtables (FAT) quicker than other stored data.

Non-Interdigitated Driver Design

However, the designs depicted in FIGS. 13A through 18B all presume thatnon-interdigitated drivers are used. FIG. 19 depicts one possible layoutof the X₀ driver set 605 that would make it possible to fit the linedrivers within the necessary line pitch. Each driver would beresponsible for supplying voltage to a specific conductive array line(or group of conductive array lines if the driver set supplied voltageto multiple memory layers). Therefore, if there were 256 conductivearray lines on the X₀ layer 425, there would be 256 drivers in the X₀driver set 605. The drivers deliver voltage to the conductive arraylines through metal interconnects 1905. Each metal interconnect 1905would preferably be fabricated to be the same width as the conductivearray lines. Although each driver in FIG. 19 spans four metalinterconnects, stacking the drivers four deep ensures that the entire X₀driver set 605 fits within the required parameters. In general terms, ifa line driver group contained N line drivers, and if a memory cell wasfabricated to a width of W, then the line driver group could not have awidth greater than N×W if the drivers were to fit completely underneaththe cross point array.

It should be appreciated that additional peripheral circuitry may beneeded in addition to the drivers. For example, the drivers mightrequire access to both a primary decoder and a secondary decoder, asdescribed in copending application Ser. No. 10/330,965, filed Dec. 26,2002, which has already been incorporated by reference. The primarydecoder is capable of selecting a single group of, for example, eightdrivers 1910 (but not a specific driver) and the secondary decoder iscapable of selecting a single driver 1915 (but not a specific group ofeight drivers 1910). Although the secondary decoder connections are notshown, the primary decoder activates the group of eight drivers througha metal interconnect line 1920. If the primary decoder selected a groupof, say, sixteen drivers, then the metal interconnect line 1920 could betied to a metal interconnect line of another set of eight drivers. Thesecondary decoder would then select a single driver out of the group ofsixteen drivers. If there was room underneath the cross point array(e.g., the configuration shown in FIG. 14B), at least some of theadditional peripheral circuitry could be placed underneath the crosspoint array.

FIG. 20A depicts one possible driver schematic in which the driver 1915requires access to a primary decoder, a secondary decoder, and ground.The driver 1915 is made up of a pass device (one p-channel transistor2005 and one n-channel transistor 2010) and a transistor 2015 to ground.The transistor to ground 2015 can be either an n-channel transistor or ap-channel transistor, depending upon how it is connected to the passdevice.

FIG. 20B depicts one layout of the three transistors 2005, 2010, and2015 that make up the driver 1915. Since one of the transistors 2005 ofthe pass device is a p-channel transistor, it must be fabricated in aportion of the semiconductor substrate that is p-doped 2020. In order toconserve space, each transistor 2005, 2010, and 2015 shares a node 2025,2030, and 2035 with a transistor from another driver 1925 in the groupof eight drivers 1910. The nodes can be shared because the entire groupof eight drivers 1910 receives the same input 1920 from the primarydecoder into each pass device, and the third transistor in each driveris always tied to ground.

Making the various connections can be accomplished through well-knownprocess steps. FIG. 21A depicts the driver 1915 after a first metallayer is deposited that provides vertical connections 2105, 2110, 2115,and 2120 from the driver to other peripheral circuitry and/or otherdrivers. The transistor to ground 2015 can share its vertical connection2115 to ground with every grounded transistor in the same horizontalposition as the drivers 1915 and 1925. However, in order to reduce loadalternative layouts may not tie all the drivers together. Similarly, thegates of each transistor 2005, 2010 and 2015 in the driver 1915 thattakes inputs from the secondary decoder can share their verticalconnections 2105, 2110, and 2115 with the gates of other drivers thathave the same position in relation to the group of eight drivers 1910.Although not shown in FIG. 21 A, the gate of the grounded transistor2015 and the gate of the p-channel transistor 2005 are driven by thesame input from the secondary decoder as shown in FIG. 20A.

Although a single via 2125, 2130, 2135, and 2140 is shown connectingeach of the transistors 2005, 2010, and 2015 to their correspondingvertical connections 2105, 2110, 2115, and 2120, it should beappreciated that multiple vias might be used in order to improveperformance. Similarly, different designs might use vertical connections2105, 2110, 2115, and 2120 that are wider than a single feature size inorder to similarly improve performance.

FIG. 21B depicts the driver 1915 after a second metal layer isdeposited. The second metal layer provides the horizontal connections2145, 2150, and 1920 that make the logical connections between the threetransistors 2005, 2010, and 2015. The transistors 2005 and 2010 in thepass device are tied together with two horizontal connectors 2145 and1920. Additionally, the horizontal connector 1920 that connects thenodes 2025 and 2030 of the pass device that is common to the two drivers1915 and 1925 extends beyond the driver 1915 boundaries to the otherdrivers in the group of eight drivers 1910 and carries signals from theprimary decoder.

Horizontal connector 2150 also connects the transistor to ground 2015 tothe pass device transistors 2005 and 2010. In one embodiment, connector2145 and connector 2150 are the same line, ensuring that all threetransistors 2005, 2010, and 2015 are tied together. The horizontalconnector 2150 is preferably the same line pitch as the conductive arraylines because the horizontal connector 2150 is the beginning of themetal interconnect that joins the driver 1915 to the appropriateconductive array line. The other metal interconnects from the otherdrivers in the group of eight drivers 1910 are also formed on the secondmetal layer. Although other embodiments could use more metal layers,such designs would typically result in a more costly fabricationprocess.

The last stage 1930 uses a slightly different layout than the previousthree stages. Since the circuitry is laid out in the pitch of 8 arraylines, which is intended to be minimum pitch, and the circuitry drives 8conductive array lines, there is no space left for the line 1920 thatdrives the common nodes of the pass device. FIG. 22 depicts the layoutfor the last stage 1930 of drivers. The gates 2205, 2210, 2215, and 2220of the pass device 2225 are lengthened with extensions 2230, 2235, 2240,and 2245. Although lengthening the pass device 2225 increases the widthof the last stage 1930, such lengthening allows for the first metallayer to be used for both horizontal and vertical connections within thepass device 2225

FIG. 23A depicts the last stage 1930 after a first metal layer isdeposited. A horizontal connector 2305 can directly connect the commonnode of the pass device 2225 because the vertical connections 2310,2315, 2320, and 2325 lie on the exterior of the pass device 2225.Additionally, the horizontal connector 2305 extends to the beginning ofa via 2330, which is how the last stage 1930 receives its input 1920from the primary decoder.

FIG. 23B depicts the last stage 1930 after the second metal layer isdeposited. The input 1920 from the primary decoder delivers its signalto the pass device 2225 through the via 2330, which connects to thehorizontal connection 2305 on the first metal layer. Allowing theprimary decoder input 1920 to drop to the first metal layer makes anadditional line available on the second metal layer, such that all eightmetal interconnects 1905 from the group of eight drivers 1910 fit on theminimum line pitch.

Concluding Remarks

Although the invention has been described in its presently contemplatedbest mode, it is clear that it is susceptible to numerous modifications,modes of operation and embodiments, all within the ability and skill ofthose familiar with the art and without exercise of further inventiveactivity. For example, the proceeding description largely described astacked cross point array 400 with four memory layers, the concepts canbe applied to stacked cross point arrays with any number of memorylayers. FIG. 24 depicts a configuration of a stacked cross point array2300 that uses eight memory layers 2405, 2410, 2415, 2420, 2425, 2430,2435, and 2440. All of the x-direction conductive array line layers2445, 2450, 2455, 2460, and 2465 are connected to one of two thrus 2470or 2475, and, therefore, share decoding logic. Each of the y-directionconductive array line layers 2480, 2485, 2490, and 2495, however, aredriven by their own decoding logic. Table 2 shows the combinations ofx-direction conductive array lines and y-direction conductive arraylines that are necessary to activate a memory element on a specificmemory layer.

TABLE 2 X₀, X₂, X₄ X₁, X₃ Y₀ Y₁ Y₂ Y₃ Memory Layer x x ML₀ x x ML₁ x xML₂ x x ML₃ x x ML₄ x x ML₅ x x ML₆ x x ML₇

Accordingly, that which is intended to be protected by Letters Patent isset forth in the claims and includes all variations and modificationsthat fall within the spirit and scope of the claims.

1. A cross point array driver set, comprising: a first plurality of linedrivers that are not interdigitated, each line driver configured todeliver a signal; a first plurality of contiguous conductive lineselectrically coupled to the first plurality of line drivers, eachconductive line configured to carry the signal delivered by a linedriver from the first plurality of line drivers; a second plurality ofline drivers that are not interdigitated, each line driver configured todeliver a signal; and a second plurality of contiguous conductive lineselectrically coupled to the second plurality of line drivers, eachconductive line configured to carry the signal delivered by a linedriver from the second plurality of line drivers; wherein the firstplurality of contiguous conductive lines are electrically coupled to thesecond plurality of contiguous conductive lines through electricalconnections that are arranged such that each conductive line from thefirst plurality of contiguous conductive lines share only a singledirect electrical connection with a conductive line from the secondplurality of contiguous conductive lines, whereby indirect electricalconnections are leakage paths.
 2. The cross point array driver set ofclaim 1, wherein: the electrical connections have impedances that can bein different states; and if the voltage drop across an electricalconnection is at a first level and the impedance is in a low state, thenthe impedance will increase to a high state; and if the voltage dropacross an electrical connection is at a second level and the impedanceis in the high state, then the impedance will decrease to the low state.3. The cross point array driver set of claim 2, wherein if the voltagedrop across the electrical connection is at a third level, then thestate can be determined without substantially increasing or decreasingthe impedance.
 4. The cross point array driver set of claim 1, wherein:the electrical connection exhibits a first resistance regime for acertain range of voltages and a second resistance regime for voltagesabove and below that range, the first resistance regime being lessconductive than the second resistance regime.
 5. The cross point arraydriver set of claim 1, wherein: the first plurality of line drivers thatare not interdigitated has a width; and the first plurality ofcontiguous conductive lines has a width that is substantially equal tothe width of the first plurality of line drivers; whereby both widthsare orthogonal to a general direction of the first plurality ofcontiguous conductive lines.
 6. The cross point array driver set ofclaim 4, wherein: the second plurality of line drivers that are notinterdigitated has a width; and the second plurality of contiguousconductive lines has a width that is substantially equal to the width ofthe second plurality of line drivers; whereby the width of the secondplurality of line drivers and the width of the second plurality ofcontiguous conductive lines are in the general direction of the firstplurality of contiguous conductive lines.
 7. The cross point arraydriver set of claim 1, further comprising: a third plurality of linedrivers that are not interdigitated, each line driver configured todeliver a signal; and a third plurality of contiguous conductive lineselectrically coupled to the third plurality of line drivers, eachconductive line configured to carry the signal delivered by a linedriver from the third plurality of line drivers; wherein the secondplurality of contiguous conductive lines are electrically coupled to thethird plurality of contiguous conductive lines through electricalconnections that are arranged such that each conductive line from thesecond plurality of contiguous conductive lines share only a singledirect electrical connection with a conductive line from the thirdplurality of contiguous conductive lines; whereby the cross point arraydriver set is associated with a stacked cross point array.
 8. A driverset comprising: a plurality of x-direction line driver groups formed ona substrate and having a width W₁ long a y-direction, each x-directionline driver group having a plurality of x-direction line drivers, eachx-direction line driver configured to, depending on a control signal,output one of several input signals; a plurality of x-directionconductive lines that receive the outputs of the plurality ofx-direction line driver groups; a plurality of y-direction line drivergroups formed on the substrate and having a width W₂ along anx-direction, each y-direction line driver group having a plurality ofy-direction line drivers, each y-direction line driver configured to,depending on a control signal, output one of several input signals; anda plurality of y-direction conductive lines that that receive theoutputs of the plurality of y-direction line driver groups; wherein theplurality of x-direction conductive lines are electrically coupled tothe plurality of y-direction conductive array lines through electricalpaths in the z-direction, each x-direction conductive line intersectingwith a row of the electrical paths in the x-direction and eachy-direction conductive line intersecting with a column of the electricalpaths in the y-direction; wherein the plurality of x-directionconductive lines has a width W₃ along the y-direction at theintersections, whereby W₃ includes both widths of the individualx-direction conductive lines and widths of any separations between thex-direction conductive lines; wherein the plurality of y-directionconductive lines has a width W₄ along the x-direction at theintersections, whereby W₄ includes both widths of the individualy-direction conductive lines and widths of any separations between they-direction conductive lines; wherein the x-direction, the y-directionand the z-direction are all substantially orthogonal to each other;wherein W₁ is approximately equal to W₃; and wherein W₂ is approximatelyequal to W₄.
 9. The driver set of claim 8 wherein the several inputsignals that each x-direction line driver is configured to outputincludes a write high signal, a write low signal and a read signal. 10.The driver set of claim 9 wherein the write high signal is a voltagepulse and the write low signals is a voltage pulse of opposite polarityto the write high voltage pulse.
 11. The driver set of claim 8, wherein:the plurality of x-direction line driver groups are interdigitated. 12.The driver set of claim 8, wherein: the plurality of x-direction linedriver groups are not interdigitated.
 13. The driver set of claim 8,further comprising: a second plurality of x-direction line driver groupsformed on the substrate, each second x-direction line driver grouphaving a plurality of second x-direction line drivers, each secondx-direction line driver configured to, depending on a control signal,output one of several input signals; a plurality of second x-directionconductive lines that receive the outputs of the plurality of secondx-direction line driver groups; and wherein the second plurality ofx-direction conductive lines are electrically coupled to the pluralityof y-direction conductive array lines through electrical paths in thez-direction, each second x-direction conductive line intersecting with arow of the electrical paths in the x-direction and each y-directionconductive line intersecting with a column of the electrical paths inthe y-direction.
 14. A cross point memory array driver set, comprising:a plurality of contiguous conductive lines, each contiguous conductiveline electrically coupled to both a line driver and a row of memorycells, each contiguous conductive line being configured substantiallyorthogonally to another contiguous conductive line and a column ofmemory cells, and each memory cell being fabricated to a width, W; and aplurality of line drivers circuits, each line driver circuit beingconfigured to deliver a voltage to a memory cell along at least one ofthe plurality of contiguous conductive lines, and the plurality of linedriver circuits having N line driver circuits and a width that is notgreater than N×W.
 15. The stacked cross point memory array driver setrecited in claim 14, wherein the voltage can cause a memory cell to bewritten to or erased.
 16. The stacked cross point memory array driverset recited in claim 14, wherein the plurality of line drivers isinterdigitated.
 17. The stacked cross point memory array driver setrecited in claim 14, wherein the plurality of line drivers isnon-interdigitated.
 18. The cross point array driver set of claim 14,wherein: the memory cell exhibits a first resistance regime for acertain range of voltages and a second resistance regime for voltagesabove and below that range, the first resistance regime being lessconductive than the second resistance regime.